In the production of ductile iron, a furnace of induction or cupola type is charged normally with 50% scrap returns of ductile or other iron from the plant, 48% of selected steel scrap and 2% graphite for carbonization. Depending on availability, pig iron scrap might be charged, and the final product quality will be a function of the raw material.
The above mentioned raw materials charging practices for the production of ductile iron are disadvantageous to obtain a uniform and sufficiently high product quality due to the inconsistency of the chemical analysis of the charge.
For producers of ductile iron, the ideal raw material is an iron of convenient and highly uniform chemical composition, free of residual elements which bring secondary detrimental effects on the microstructure and properties of the final products. Additionally, if the iron is handled in its solid phase, it is economically advantageous to do it with the optimum weight and geometry for its transportation, handling, charging to the furnace and melting conditions where the ductile iron is to be produced.
The proposed method for fabricating low residual iron metal avoids systematically the introduction of tramp elements brought by the initial scrap charge.
More specifically, microstructure and properties are affected by zinc, lead, arsenic, bismuth and cadmium which deteriorate the morphology of the graphite modules and favor the formation of flakes or vermicular graphite.
Other critical residual elements such as molybdenum, chromiun, boron, etc. result in the formation of detrimental carbides requiring an extra final heat treatment for the cast piece, as is the case in many foundries.
The above mentioned aspects concern with the starting material involved in the production of a high quality iron, particularly ductile iron whose chemical analysis and condition in liquid phase before solidification, are critical for the proper microstructure and final mechanical properties.
Also microstructure (particularly nodules morphology and consequent properties), can be affected by improper inoculation procedures.
In common processes for making ductile iron, normally, liquid iron to be transformed into ductile iron is tapped either from a cupola furnace or an induction furnace and some times from an electric furnace.
These processes have advantages and disadvantages; the induction furnace is very useful in making ductile iron but in any form the raw material analysis is in essence inherited to the final product. It is possible to charge high quality starting scrap to reach in the induction furnace the critical condition of liquid bath for making ductile iron. For instance, It is possible to charge small ingots produced by the method herewith proposed, in which the basic raw material is sponge iron; but sponge iron could not be charged directly into the induction furnace since up to date it has not been possible to handle the slag produced in the smelting of sponge iron.
The cupola furnace follows the same behavior regarding the inheriting effect from raw material, however it is possible to get an excellent liquid for making high quality ductile iron by a very drastic control in raw materials. Furthermore, the cupola furnace operation and product quality also depend on the coke properties (size and uniformity). In case of low availability of high quality coke, this presents an obstacle for making high quality ductile iron. A cupola furnace can be charged with sponge iron or prereduced pellets for improving the chemical analysis, but again it fails to reach a good condition in operation, as pellets fall through the coke bed making the operation erratic. Also, as in the case of the induction furnace, the increasing slag makes it difficult to remove it after certain limits of slag volume produced.
The method for producing controlled chemical analysis liquid iron herewith proposed, overcomes the mentioned problems and permits the production of high quality ductile iron. In the proposed method, the process for iron bearing materials melting is carried out in a converter vessel and therefore it is possible to handle a high slag volume. As self sufficient in energy it is also possible to melt all the charge from prereduced iron in any form. Regarding specific requirements for energy source as is the case of special coke for the cupola, the proposed process is more flexible as it injects any solid carbonaceus materials in form of fines therefore not depending on certain materials such as coke. All conventional methods for producing ductile iron have inoculation as a common problem due to the volatile properties of magnesium. In the present process, magnesium is injected through the bottom of the converter, which increases the efficiency of inoculation.